Photo-sensitive devices employing photo-conductive coatings



y 14, 1963 H. CASSMAN 3,383,244

PHOTO-SENSITIVE DEVICES EMPLOYING PHOTO-CONDUCTIVE COATINGS Filed June 2, 1964 United States Patent 3,383,244 PHOTO-SENSITIVE DEVICES EMPLOYING PHOTO-CONDUCTIVE COATINGS Harry Cassman, Ealing, London, England, assignor to Electric 8: Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Filed June 2, 1964, Ser. No. 371,971 Claims priority, application Great Britain, June 6, 1%3, 22.,500/63 9 Claims. (Cl. 117-210) ABSTRACT OF THE DISCLOSURE This specification describes a photo-conductive layer to be used in for example a vidicon type pick-up tube, including a porous layer of photo-conductive material in contact with the signal plate and a semi-porous layer of photo-conductive material on said porous layer. Said porous and semi-porous layers are formed by evaporating photo-conductive material from an evaporator on to a support in gaseous atmospheres under such conditions that in the case of the formation of the porous layer the distance from the evaporator to the surface is several hundred times the mean free path of molecules of the photo-conductive material, and in the case of the formation of the semiporous layer the distance from the evaporator from the surface is about twenty times the mean free path of the molecules of the photo-conductive material.

This invention relates to photo-sensitive devices comprising photo-conductive coatings such as are employed as the targets of photo-sensitive television pick-up tubes. The invention relates also to an improved method of forming such coatings.

It has previously been proposed, in the formation of targets for television pick-up tubes known as vidicons, to form the photo-sensitive target as a porous layer on a transparent electrically conductive signal electrode. A porous layer is one which is of a nature similar to that obtained by the deposition by evaporation of a material in the presence of a gas at such pressure as to cause random collision of the particles so that the deposited layer has superimposed particles randomly disposed and having voids therebetween. It has also been proposed to form a composite target layer by the deposition in contact with a porous layer of a solid layer of the same or a different photo-conductive material. A solid layer is one which has a nature similar to a layer formed by the deposition by evaporation of a material in vacuo having a pressure so low that the mean free path of the molecules of the material to be evaporated is of the same order or greater than the distance between an evaporator from which the material is evaporated and a support on which the material is deposited. Such a solid layer when deposited on a smooth surface would have substantially no voids between the particles.

One object of the present invention is to provide an improved photo-sensitive device comprising superimposed layers of photo-conductive material.

According to one aspect of the present invention there is provided a photo-sensitive device having a target comprising a porous layer of photo-conductive material, a semi-porous layer of photo-conductive material deposited on said porous layer, a translucent support carrying said layers on one side of said support and a translucent electrical conducting signal plate deposited on one side of said support.

According to another aspect of the present invention there is provided a television pick-up tube having a target including an electrically conducting elect-rode having photoconductive coating thereon, said coating comprising 3,383,244 Patented May 14, 1968 a porous layer and a semi-porous layer of photo-conductive material in contact with one another.

In the above statements the porous and semi-porous layers are such as would be formed by evaporating photoconductive material from an evaporator on to a surface of a support in a gaseous atmosphere under such conditions that the distance from the evaporator to the surface on which the layer is formed is greater than the mean free path of the molecules of the photo-conductive material; in the case of the formation of said porous layer said distance is several hundred times said mean free path and in the case of said semi-porous layer said distance is about twenty times said mean free path.

The term porous layer which is used herein and in the claims is intended to mean a layer comprising superimposed particles randomly disposed and having voids therebetween and of a nature substantially similar to that obtained by the deposition by evaporation of a material in the presence of a gas at such a pressure that the mean free path of the molecules of the evaporated material is many times less than the distance between the evaporator, from which the material is evaporated, and the support on which the material is deposited. A semi-porous layer is one in which the superimposed particles are smaller and have smaller voids between them than in a porous layer, being of a nature similar to that obtained by the deposition by evaporation of a material in the presence of a gas at such pressure that the mean free path of the molecules of evaporated material is substantially greater than that required to produce the porous layer but is nevertheless such that collisions occur between evaporated molecules. By way of example, both layers may be evaporated in the presence of an inert gas such as argon the porous layer being formed at a pressure of the order of 1.0 mm. of mercury and the semi-porous layer being formed at a pressure of 0.03 mm. of mercury.

In order that the invention may be clearly understood. and readily carried into effect it will now be more fully described with reference to the accompanying drawings, in which:

FIGURE 1 shows a longitudinal sectional view of one form of a photo-sensitive device according to an embodiment of the invention,

FIGURE 2 shows apparatus used in the manufacture of the photo-sensitive target of the device of FIGURE 1, and

FIGURE 3 shows, on an enlarged scale, a section of said photo-Sensitive target.

In FIGURE 1 of the drawings the invention is shown, by way of example as applied to a photo-conductive target of a television pick-up tube. The tube comprises a tubular glass envelope 1 having at one end an optical window 2 of glass or quartz hermetically sealed to the envelope 1 through the intermediary of a metal ring 3 and indium solder 4 in known manner, said window 2 supporting the photo-conductive target which will be described in detailed hereinafter. At the other end of the envelope 1 there is provided a glass base 5 having metal contact members 6 hermetically sealed therethrough and arranged in a circular array around a pump stem 7. Supported from the members 6 within the envelope 1 is an electron gun which as shown comprises a thermionic cathode 8 with its associated heating element 9 and a so-called grid electrode 10 which during operation may be maintained at a negative potential of 0-100 volts with respect to cathode 8. The grid 10 is followed by an anode structure 11, said anode 11 having spaced apart apertures 12, 13 of different diameters to reduce the lateral components of the beam in known manner and which may in operation be maintained at a potential of 300 volts positive with respect to cathode 8. The inner surface of the window 3 is provided with a substantially transparent electrically conducting coating 14 forming a signal plate which is electrically connected to the ring 3 and to which is applied in operation a potential which may be up to 150 volts positive with respect to the operating potential of the cathode 8 which may be maintained at zero volts. The coating 14 has deposited thereon a layer 15 of porous photo-conductive material, and deposited on said layer 15 is a layer 16 of semi-porous photo-conductive material.

Adjacent to and facing the target thus formed is provided a metal mesh electrode 17 and disposed between the electrode 17 and the anode 13 is a cylindrical electrode 18 which has fingers 19 whereby it is located axially within the envelope 1. The mesh 17 functions in the operation of the tube as an ion trap and is maintained usually at a slightly higher potential than the cylindrical electrode 18. The electrode 18 extends over a considerable area of the internal wall of the envelope 1 and functions as a wall anode and is maintained at a positive potential of, for example, 280 volts with respect to the cathode 3.

In this particular embodiment the photo-conductive target is formed on the window 2 before said window 2 is sealed to the envelope 1. The electrically conducting signal plate 14 is formed on the window 2 in known manner by spraying said window 2 whilst hot with a solution of a tin salt. The window 2, which may be 1 inch in diameter is mounted in a suitable container such as a bell jar 20 which can be evacuated via a pump stem 21, and in which is mounted an evaporator 22 in the form of a boat disposed with its open side facing the signal plate 14 and containing a suitable photo-conductive material 23 for example antimony tri-sulphide. The evaporator 22 may be disposed with its opening positioned at from 25 to mms. from the signal plate 14 suitable means being provided for raising the temperature of the material 23 to its evaporation point. A mask 24 is included so that after evaporation a narrow peripheral ring of the conducting signal plate 14 remains uncoated.

The container 20 is evacuated and thereafter an inert gas, preferably argon, is introduced therein to a pressure of about 1 mm. of mercury and the material 23 is heated to the evaporation temperature so that particles of said material are deposited on the exposed surface of the electrically conducting signal plate 14. This evaporation is continued until a suitable thickness of porous layer 28 is deposited as shown in FIGURE 3, this thickness being, for example, from 1 to 2 microns. The mean free path of molecules of antimony tri-sulphide evaporated in argon at a pressure of 1 mm. Hg is estimated to be .05 mm. so there will be abundant chances of the molecules colliding together to form relatively large particles which deposit on the surface as a porous layer 28.

Processes for forming porous layer just described are in general well known in the art of producing targets for vidicon tubes.

Although such porous layers of antimony tri-sulphide alone can provide targets which are photo-sensitive with a relatively short capacitative lag, they have the disadvantage of low sensitivity and a colour response which is peaked towards the blue end of the spectrum. Another disadvantage with this method is the pronounced tendency to exhibit a negative burnt-in image after quite short exposures. To increase the sensitivity and render the colour response more panchromatic it has been proposed to evaporate a solid layer of photo-conductive material onto the porous layer. This process is relatively successful with vidicon tubes made by the in-situ method in which the whole of the processing of the target is performed with the target support member sealed to the envelope of the tube and with the electron gun sealed in position. However, it is less successful, particularly in respect of sensi tivity, when a similar method is employed for pre-fabricating targets as separate items which can subsequently be sealed to the open end of a glass cylinder containing the electron gun thereby to complete t e t be assembly- According to the invention therefore, after a porous layer is deposited on the target support member, another layer defined as a semi-porous layer is deposited on the porous layer. The semi-porous layer is obtained by evaporating a further quantity of antimony tri-sulphide in an inert gas such as argon at a much lower pressure to deposit the evaporated material onto and in contact with the surface of the porous layer.

In the example of the invention being described, the semi-porous layer is produced by reducing the gas pressure for the second evaporation to .03 mm. Hg, the layer being deposited to a thickness of 0.5 The mean free path of the evaporated molecules at this pressure is estimated to be 1.5 mm. and as the evaporation is performed from the same evaporator mounted at the same position as that employed for the above mention formation of the porous layer then it will be seen that the chances of collisions occurring in their passage from the evaporator to the support on which they are deposited is considerably less than when the gas pressure is such as to provide a porous layer. A porous layer obtains its character from the deposition of relatively large particles and the evaporation of the same material at a lower gas pressure will produce a layer still having a somewhat porous character because the mean free path in the gas, though greater than during the deposition of the porous layer, is nevertheless many times less than the distance of the evaporator from the support, so that collisions between molecules of evaporated material do occur. However, such collisions are many fewer in number so that the particles are of much smaller size than in the porous layer.

A vidicon having a target produced by a prefabrication method as described with a porous layer and a semiporous layer has a substantially panchromatic colour response and a good overall sensitivity. Furthermore, with vidicon tubes made according to the invention the negative burn-in of images is almostly completely eliminated.

Although the invention has been described applied to the production of layers by a pre-fabrication method it can also be applied to the formation of layers wholly within the envelope of the device by an in-situ method. Furthermore, another layer, such as a thin solid layer of antimony tri-sulphide or other photo-conductive material, for example may be deposited on the electrically conducting electrode as a substrate for the porous and semiporous composite layer according to the invention.

What I claim is:

1. A television pick-up tube having a target including an electrically conducting electrode and a photo-conductive coating comprising a porous layer and a semi-porous layer of photo-conductive material in contact with one another, said layers being such as would be formed by evaporating photo-conductive material from an evaporator on to a surface of a support in a gaseous atmosphere under such conditions that the distance from the evaporator to the surface on which the layer is formed is greater than the mean free path of the molecules of the photoconductive material, in the case of the formation of said porous layer said distance is several hundred times said mean free path and in the case of said formation of semiporous layers said distance is about twenty times said mean free path.

2. A photo-sensitive device having a target comprising a porous layer of photo-conductive material, a semiporous layer of photo-conductive material deposited on said porous layer, a translucent support carrying said layers on one side of said support and a translucent electrically conducting signal plate deposited on said one side of said support, said layers being such as would be formed by evaporating photo-conductive material from an evaporator on to the surface of said support in a gaseous atmosphere under such conditions that the distance from the evaporator to the surface on which the layer is formed is greater than the mean free path of the molecules of the photo-conductive material, in the case of the formation of said porous layer said distance is several hundred times said mean free path, and in the case of the formation of said semi-porous layer said distance is about twenty times said mean free path.

3. A television pick-up tube including a photo-conductive target formed by evaporating photo-conductive material from an evaporator in an atmosphere of a gas at a pressure of 1 millimetre of mercury to deposit a layer of said material on a translucent support in porous form, and subsequently evaporating photo-conductive material in an atmosphere of a gas at a pressure of 0.03 millimetre of mercury to deposit a layer of said material on said porous layer in semi-porous form the spacing of said evaporator from said support being in the range 25 to 40 millimetres.

4. A tube according to claim 1 wherein in the case of the formation of said semi-porous layer said distance is in the range of between 16 and 27 times said mean free path.

5. A tube according to claim 1 wherein said photoconductive material is antimony trisulphide.

6. A tube according to claim 5 wherein said porous layer is one to two microns thick and said semi-porous layer is 0.5 micron thick.

7. A device according to claim 2 wherein in the case of the formation of said semi-porous layer said distance is in the range between 16 and 27 times said mean free path.

8. A device according to claim 2 wherein said photoconductive material is antimony trisulphide.

9. A device according to claim 3 wherein said porous layer is one to two microns thick and said semi-porous layer is 0.5 micron thick.

References Cited UNITED STATES PATENTS 2,829,074 4/1958 Lubszynski l17-34X 2,900,280 8/1959 Lubszynski et al. 117 -106X ALFRED L. LEAVITT, Primary Examiner.

J. H. NEWSOME, Assistant Examiner. 

